1. Field of the Invention
The present invention relates to an image forming apparatus capable of, according to an electrophotographic method, capable of forming an electrostatic latent image with a laser beam scanning on the photosensitive drum, and developing a visible image on a recording medium by applying toner to the electrostatic latent image.
2. Description of the Related Art
An electrophotographic image forming apparatus forms an electrostatic latent image on a photosensitive member by exposing the photosensitive member with a light flux output from an exposure device. In particular, it is desirable that scanning lines (the moving locus of a light flux) on the photosensitive member should be straight with no inclination. If the exposing position deviates from an ideal position, causing the scanning lines to be inclined or curved, the image quality will deteriorate. In the case of an image forming apparatus that forms multicolor images by using toners of different colors, if scanning lines on respective image bearing members have different amounts of deviation, so-called color misregistration becomes visually noticeable.
In Japanese Patent Application Laid-Open No. 2004-170755, such a method is discussed as adjusting timing for writing scanning lines and adjusting the amount of exposure by measuring inclinations and curvatures of the scanning lines for each image height (the main scanning position) by using a number of registration detecting sensors arranged along the main scanning direction (along the axial direction of the photosensitive drum).
The prior art described above is fairly effective in improving color misregistration. However, if the inclination of the scanning line, which is one of the causes for color misregistration, is corrected, partial magnification of a scanning line changes. Therefore, it becomes necessary to correct the partial magnifications by forming a registration pattern again. When this correction process is executed, the image forming process is interrupted. Therefore, if the correction process is executed repeatedly, downtime, in which images cannot be formed, increases.
Furthermore, in the prior art, to correct the partial magnification with high accuracy, it is required to install the sensors in at least three positions along the main scanning direction. In that case, not only the number of components to be built in but also the number of patterns to be formed and the amount of signal processing to be executed will increase.